Known pulverized-coal systems pulverize coal, deliver it to the fuel-burning equipment, and accomplish complete combustion in the furnace with a minimum of excess air. The system operates as a continuous process and, within specified design limitations, the coal supply or feed can be varied as rapidly and as widely as required by the combustion process.
A small portion of the air required for combustion (15 to 20% in current installations) is used to transport the coal to the burner. This is known as primary air. In the direct-firing system, primary air is also used to dry the coal in the pulverizer. The remainder of the combustion air (80 to 85%) is introduced at the burner and is known as secondary air.
All coals, when exposed to air, undergo oxidation even at room temperature. This tendency varies with coal type: anthracite and semi-anthracite, for example, are little affected whereas many bituminous coals are particularly liable to absorb and combine with oxygen. The process of oxidation continues with increasing rapidity as the temperature rises. Heat is generated which, if allowed to accumulate, could result in thermal decomposition and ignition of the coal. Volatile components of the coal, such as methane and related compounds, are released during the decomposition. Accumulation of these gaseous materials may be ignited at fairly low temperatures and rapidly propagate fire or explosion.
Spontaneous combustion of coal is dependent on a sufficient supply of oxygen to maintain the reaction and on the surface area exposed. Coals with a high surface area, due to small particle size, as in pulverized coal fuel, are particularly liable to self heating. This problem is of special significance to the safe operation and performance of industrial coal pulverizers. Spontaneous combustion may result in deterioration in the quality of the coal, in damage to the power plant, and in certain cases, for example, where critical concentrations of coal dust are involved, may provide the ignition source for an explosion.
Present systems for fire detection in industrial coal pulverizers use either thermocouples to measure the rise in outlet temperature of the pulverizing mill or infrared gas analyzers to detect the buildup of CO produced in the mill.
Thermocouples or RTD's are normally part of the control system for mill operation. However, they are a relatively insensitive means for detecting pulverizer fires. At best, they warn of impending trouble only a few minutes before it actually occurs, and in some cases, do not even detect a significant temperature rise before a fire or explosion is evident. The ineffectiveness of thermocouples and RTD's in this application is due, in part, to the shielding used to protect them from the corrosive coal particles. Shields reduce heat conduction, slowing response time.
Actual CO measurements are also used for fire detection in coal pulverizers since that CO buildup is related directly to the oxidation rate of coal. Infrared gas analyzers are used to compare the CO content of the incoming and outgoing mill air and in effect, the amount of CO produced in the mill. Currently available infrared gas analyzers require extensive filtering and dehydration of the gas sample extracted from the mill, to prevent interference by water vapor and particulate matter. Due to the high cost and maintenance requirements of infrared absorption analyzers, it is the usual practice to use one analyzer for several measurement points. Continuous measurement of each mill is not provided, thus, slowing response time. Nevertheless, this provides an improvement over the thermocuple and RTD method described. Additional problems occur, at some power plants, where appreciable concentrations of CO can be found in the air supply to the mill. Since in such plants CO in the boiler flue gases is transferred to the combustion air via the regenerative air heater and it thus becomes necessary to provide an analysis of the air entering the mill.
Thus, it is seen that an accurate and reliable safety system was required for coal pulverizers which would provide an early warning of impending safety problems in coal pulverizers.